This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to 9904595-7 filed in Sweden on Dec. 15, 1999; the entire content of which is hereby incorporated by reference.
The present invention relates to a power transistor module intended for radio frequency applications, particularly for use in an amplifier stage in a radio base station or in a ground transmitter for television or radio, to a power amplifier comprising said power transistor module and a method in the fabrication of the power amplifier.
Power transistor modules for amplification at high frequencies have to comply with a number for detailed requirements as regards power amplification, raggedness, break-down voltages, noise, distortion, capacitances, input and output impedences, etc. at a specified feed voltage and operation frequency. The demand on output power vary from a few watts to several hundred watts, in the latter case several components connected in parallel in a module being utilized. Power transistors operate at high signal levels and thus high current densities. The operation frequency is within the radio and microwave frequencies.
The power transistor modules are the critical components in power amplifiers in radio transmitters. The performance of the modules is the limiting factor as regards output power, efficiency and reliability.
In FIG. 1 is shown, schematically in cross-section, a power transistor module 1 mounted at a heat sink 3 and electrically connected to conductor pattern (not shown) on a circuit board or a printed circuit board 5 according to known practice.
Module 1 comprises a rectangular electrically conducting and heat conducting bottom plate or flange 7, preferably of a metallic material, onto the upper surface of which are mounted a transistor chip 9 and typically a first 11 and a second 13 capacitor chip. Further, module 1 comprises an insulating, preferably ceramic, frame arrangement or isolator 15 mounted on the upper surface of flange 7 so that the arrangement surrounds said chips, and a cover 17 attached to frame 15.
The transistor chip 9 is typically a chip of the LDMOS (Lateral Double Diffused Metal Oxide) type and comprises a row of blocks connected in parallel, which each comprises a large number of transistor cells connected in parallel. The higher output power that is desired, the more transistor cells are needed. In FIG. 2 is shown a schematic layout of the transistor chip 9 comprising seven blocks. In each block all gates are connected in parallel to a gate connection or pad and all drains are connected to a drain connection or pad. In the uppermost block the gate pad is indicated by 19 and the drain pad is indicated by 21. The sources have connections at the back side/bottom side of the chip, which are connected to ground, e.g. via flange 7.
High power transistors have at high frequencies very low input and output impedances. To match these to surrounding circuits on the circuit board 5 impedance matching networks are needed close to the active chip with the transistors. Usually, these matching networks are implemented by using bonding wires 23, 25, 27, 29 and capacitor chips 11, 13 inside the power transistor module 1. The bonding wires are further connected to straight butt connection contacts 31, 33, preferably of alloy 42, which are projecting from module 1 for connection to said surrounding circuits. In FIG. 2 bonding wires 25, 27 are indicated at the uppermost block of the chip. Each other block is certainly connected via its own bonding wires (not shown). Difficulties to attain tight tolerances during manufacture of the modules give rise to undesired variations in different critical electrical parameters.
The module 1 may be fastened to heat sink 3 by screws or bolts, whereafter the butt connection contacts 31, 33 are manually soldered to the surrounding circuits of circuit board 5 through soldered seams 35, 37. Flange 7 and heat sink 3 typically comprise apertures or recesses for receiving screws or bolts (not shown) for the mounting of module 1.
The matching network gives an extra contribution to the impedances before an outer impedance matching is performed on the circuit board. The placement of the transistor chip in the module is very critical, since it affects the length and form of the bonding wires and thereby the impedance matching. Length and form of the bonding wires may be difficult to define.
The low output impedance of the transistor chip is caused by the capacitance of the transistor chip together with a relatively low feeding voltage. The possibilities to achieve a good matching to the surrounding circuits are limited by these facts and by inductance in the conductors (particularly the bonding wires) between the power transistor module and the circuit board.
The impedance matching, which is required, therefore has to be performed both inside the power transistor module and outside on the circuit board. Variations in the properties of the transistor module, in the dimensions of the circuit boards and in the placement of the components will imply that a power amplifier comprising said components ought to be functionally trimmed to possess the best performance. Tolerance equalization through trimming, however, is not desirable by amplifier manufacturers. Manufacture without trimming will, however, rarely provide for optimal performance but results is discarding of transistor modules.
Further, high power transistor modules of the above kind cannot be surface soldered (i.e. machine soldered) together with other surface soldered circuits on the circuit board and thereafter be mounted, i.e. fastened by screws, to a heat sink, since the modules are mechanically stressed during the screwing. The force arisen hereby is directly transferred to the soldered seams through the butt connector contacts, such that the soldered seams will be damaged, e.g. they will crack. This involves that the transistors have to be soldered manually as indicated, which is time-consuming, costly and results in large quality variations.
Due to the straight butt connector contacts, which are very rigid, the movements, which arise due to the temperature variations, will direct affect the soldered seams between the module and the circuit board. During these stresses the solder will plasticize and relax, which results in congruence displacements. When this is repeated sufficiently many times the soldered seams start to crack, which constitutes the main source of failure during operation. The lifetime of an amplifier will in this case be 5-10 years instead of the desired 30 years.
Existing capsules for transistors have preferably a metallic flange and a ceramic isolator. Both the flange and the isolator must have a certain thickness due to purely mechanical reasons and the transistor module will therefore be thicker than the circuit board (which normally is 0.8 mm). A recess must therefore be milled in the heat sink in order to be able to lower the flange of the transistor module, see FIG. 1. This involves increased manufacturing expenditures and a risk for irregularities in the bottom of the recesses. Such an irregular mounting surface results in a risk that the capsules are bent during the fastening by screws, so that the chips crack. Further, the cooling is deteriorated.
It is an object of the present invention to provide a power transistor module intended for radio frequency applications, particularly for use in an amplifier stage in a radio base station, which is lacking at least some of the problems which are associated with the prior art.
It is in this respect a particular object of the present invention to provide a power transistor module, which reduces the problems of endurance stress of the soldered seams.
It is a further object of the invention to provide a power transistor module, which may be machine soldered to a circuit board.
Further objects of the present invention will be apparent from the detailed description below.
According to a first aspect of the present invention a power transistor module is thus provided comprising a support, a power transistor chip arranged thereon, outer electrical connections projecting from the support for external connection, and inner electrical connections connected between said transistor chip and said outer connections. At least one of the outer electrical connections of the power transistor is according to the invention comprised of a first conductor pattern provided on a flexible foil.
Preferably, the flexible foil with the first conductor pattern is machine soldered to a circuit board mounted at a heat sink. In this respect the flexible foil may preferably comprise through holes at the soldering joint for the automatic soldering, which during the soldering serve as venting openings for the solvent gases originating from the soldering paste, so that the soldered seam between the foil and the circuit board will be free from pores.
The power transistor module is preferably attached, particularly fastened by screws or bolts, to the heat sink, and the flexible foil with the first conductor pattern is sufficiently flexible to substantially eliminate stresses in plasticizing of the soldered seam between the foil with the conductor pattern and the circuit board.
The flexible foil is preferably manufactured of an elastic polymer, e.g. a polyimide, and each conductor pattern is formed at least at one surface of said foil, preferably by printing or etching.
According to a further aspect of the present invention a power amplifier is provided comprising a power transistor module of the above-mentioned kind.
It is a further object of the invention to provide a method in the fabrication of said power amplifier.
According to yet a further aspect of the present invention a method in the fabrication of a power amplifier is thus provided comprising the following steps:
(1) manufacturing a power transistor module with a support, a power transistor chip arranged thereon, outer electrical connections projecting from the support and inner electrical connections connected between said transistor chip and said outer connections, said outer electrical connections being comprised of a first conductor pattern arranged on a flexible foil,
(2) electrically connecting said conductor pattern to a circuit board mounted at a heat sink and
(3) mounting said power transistor module at said heat sink.
Further, power amplifier manufactured according to the method is provided.
An advantage of the present invention is that it provides for power transistor modules and power amplifiers with improved operation reliability and longer life-time.
Yet an advantage of the present invention is that a faster, cheaper and more reliable fabrication method is achieved.
Further features and advantages of the present invention will be apparent from the following detailed description of preferred embodiments of the invention.